Where should a pronoun be placed? anywhere in the sentence somewhere before the antecedent as close to the antecedent as possible anywhere after the antecedent?

The answer is: As close to the antecedent as possible

Ifound the exercise on the internet. attached is the list mentioned above.in order for a compound to be considered optically active it can't have any axis or plan of symmetry. the molecule  shouldn't also  be superimposable on its mirror image.with such considerations, the compounds that are optically active are: -third compound on the first image is optically active; -second and third compound from the second image are optically actives.